There's this one other thing about the Jolla phone that sets it apart from the competition. In marketing terms, it's called The Other Half, the backside of the phone, which can be replaced and is 3D-printable. While the two The Other Halfs shipping with the first wave of pre-order customers have tiny RFID chips in them for communication (it instructs the phone to download a matching background and sounds), the Jolla phone also has a set of electrical contacts on the back of the exposed device - I2C.

This is one of the wildcard when it comes to Jolla - there's lots of possibilities here, such as a backplate with an additional battery, or even one with an integrated sliding keyboard. One of Jolla's engineers already added wireless charging to his backplate using I2C, to illustrate what it possible.

I'm really curious what other people are going to come up with - if at all. Right now there's probably little commercial interest to create products for The Other Half, but if Jolla manages to pick up enough steam, we might see some really cool stuff coming out of this.

Many sensors come with I2C interface, so potentially you can wire them directly to the bus, with a minimal number of external components.
Using USB would require a USB-to-bus conversion (more components on the backcover, where space is at a premium) and the energy to power the converter (more battery consumption).